(a) Field of the Invention
The present invention relates to a packet data transmission method in orthogonal frequency division multiplex access (OFDMA), and an apparatus thereof. More particularly, it relates to a packet data transmission method using transmit diversity in an uplink of a cellular system using a discrete Fourier transform spread orthogonal frequency division multiplex access (DFT-S-OFDMA) method, and a transmitting apparatus thereof.
(b) Description of the Related Art
Including a wireless local area network (WLAN), wireless broadcasting, or digital multimedia broadcasting (DMB), a fourth generation mobile communication system uses an orthogonal frequency division multiplex access (OFDMA) method for wideband high-speed data transmission.
The OFDMA method divides a frequency bandwidth of a channel into a plurality of frequency bandwidths and allocates a proper number of data bits to each bandwidth for transmission. Herein, each frequency bandwidth has a different gain and noise effect. The OFDMA method serially inputs a data stream into N parallel data rows and transmits the N parallel data rows respectively through separate subcarriers so as to increase the data rate. In this case, a discrete Fourier transform spread orthogonal frequency division multiplex access (DFT-S-OFDMA) method is used to discretely separate an OFDMA signal into a time axis and a frequency axis so as to analyze a complex signal.
A high data rate in a transmitting station and a receiving station of the OFDMA system can be realized by improving link throughputs and network capacity. In this case, the throughput can be significantly improved when the transmitting station and the receiving station respectively have multiple antennas, and thus each station transmits and receives data by using the multiple antennas. As described, a method for the transmitting station and the receiving station to transmit/receive data by using the multiple antennas is called multiple-input multiple-output (MIMO).
In an MIMO-applied OFDMA system, a transmitting station must distinguish a receiving station from other receiving stations when the corresponding receiving station attempts random access. Accordingly, a random access channel burst (RACH burst) is used to check channel estimation information including a frequency domain signature sequence or a time domain signature sequence, an identifier (ID) of a receiving station, and a resource request for call setting.
Recently, a transmitter of the OFDM system has included a transmit vector decision module and a transmit antenna decision module in order to select a specific antenna through which a RACH burst or a user packet data is transmitted among multiple antennas.
In addition, the transmit vector decision module calculates a transmit weight to be set for each antenna by using channel information calculated from a signal to noise ratio (SNR) or a code book, and the transmit antenna decision module selects an antenna unit through which the RACH burst or the user packet data is transmitting by using the transmit weight. Herein, the antenna unit includes an inverse fast Fourier transform (IFFT) unit, a cyclic prefix (CP) adder, a parallel-to-serial (P/S) converter, an intermediate frequency/radio frequency (IF/RF) converter, and an antenna.
That is, the transmitter of the OFDM system includes a multiplexer, a transmit vector decision module, a transmit antenna decision module, a plurality of IFFTs, a CP adder, a P/S converter, an IF/RF converter, and an antenna. As described, the transmitter has drawbacks including having many constituent elements, a complicated composition, an increased size of a mobile station, and an increased manufacturing cost.
In addition, although the MIMO is applied to the mobile station, the RACH burst and the user packet data are transmitted only by an antenna selected by the transmit vector decision module and the transmit antenna decision module, and therefore frequency and phase may vary at random, causing a fading effect. Accordingly, the size of a transmit signal may become greater than or smaller than an average size.
A variation speed of the fading effect may vary depending on a moving speed of the mobile station. That is, the variation speed of the fading effect is increased as the moving speed of the mobile station is increased, and the variation speed of the fading effect is decreased as the moving speed of the mobile station is decreased.
Since the variation speed of the fading effect is slow when the moving speed of the mobile station is slow, a deep fading period where the size of a signal is less than an average size is increased. During the deep fading period, a transmitted RACH signal cannot be demodulated so system performance is deteriorated.
Therefore, the transmitting station and the receiving station use all antennas when transmitting/receiving a RACH burst or user packet data to thereby use another antenna for data transmission when the deep fading effect occurs, and hence, a transmission method is required to reduce the deep fading period, and improve system performance.
While using an OFDM method, a MIMO method, and a smart antenna method, a long term evolution (LTE) method has been disclosed to increase a bandwidth that has been limited to 5 Mhz to from this value to 1.25 Mhz to 20 Mhz. With the increase of the bandwidth, the LTE method can support a data rate of 100 Mbps in a moving state and a data rate of 1 Gbps in a stopped state, and the data upload speed becomes 60 Mbps. In addition, the LTE method has a characteristic of grouping an Internet protocol (IP) network and a data network into one group, and therefore a mobile station can be simplified in an OFDM-based wireless access network.
Such an LTE method uses an OFDMA method in a downlink, and uses a DFT-S-OFDMA method in an uplink. However, the RACH burst and the user packet data are transmitted by using one antenna since a method for transmitting DFT-S-OFMDA-based uplink data by using a plurality of antennas has not been realized in the LTE method, and accordingly the deep fading may occur.